IDY25000: Hybrid-height (rho-lvls) "wind" fields

A number of raw and post-processed NWP products are available as outputs from the Australian Community Climate Earth-System Simulator (ACCESS) suite of Numerical Weather Prediction models which are run routinely by the Bureau of Meteorology's National Meteorological & Oceanographic Centre (NMOC). This document describes the content of the ACCESS-G (version APS0) IDY25000.wind.rho-lvls.YYYYMMDDHH.HHH.model series of products.

Model summary

ACCESS-G covers a global domain, has a resolution of about 80 km and is run twice per day (00Z and 12Z basetimes) out to a forecast hour of +240. This model consists of both an assimilation and forecast component.

NWP data filename convention

All NWP files in this product series have names that conform to the following convention:

IDY25000.wind.rho-lvls.YYYYMMDDHH.HHH.model.grb or IDY25000.wind.rho-lvls.YYYYMMDDHH.HHH.model.nc

Horizontal grid geometries

Latitude values in GRIB and NetCDF files are ordered North to South while longitude values are ordered West to East.

Longitudes always lie in the range 0.0 to 360.0 degrees. Longitudes can be transformed to lie between -180 and +180 degrees by subtracting 360 degrees from any longitude values greater than 180.

Vertical levels

Eta-level index values:

1 to 50 by 1

All fields lie on eta "theta" levels.
Refer to Appendix C for more information.

Time steps

Forecast hours of multi-level fields:

0 to 117 by 3, 120 to 240 by 6

Parameters

Multi-level fields

Par

NetCDF var

Steps

Units

Description

132.128

merid_wnd

All

m s-1

Meridional wind

131.128

zonal_wnd

All

m s-1

Zonal wind

Explanation of steps column terms

All

Parameter available at all time-steps

fhr...

Parameter available from fhr onward

Appendix A - Notes on specific NWP file-formats

GRIB-1

ACCESS data in GRIB-1 format is defined relative to the Bureau's local GRIB tables.
A copy of these tables in plain-text format is available for download from
the ACCESS NWP web data interface.

Local extensions are also present in the GRIB-1 message headers. The following tables summarise these.

Section 1 octet

Value

Remarks

29-30

Stash ID

16-bit UM STASH ID.

31

4V offset

Number of hours from start of 4V assimilation
window to nominal analysis time.

19

P1 period of time

Be aware that currently this is sometimes being set
incorrectly for averaged, accumulated and max/min quantities
(at certain timesteps only).

APS0 Model name

GeneratingProcessIdentifier (ID of model)

ACCESS-G

1

ACCESS-T

10

ACCESS-R

20

ACCESS-A

30

ACCESS-VT

50

ACCESS-SY

54

ACCESS-BN

58

ACCESS-AD

62

ACCESS-PH

66

ACCESS-TC

80

ACCESS-O3

90

For native (hybrid-height) level data, a GRIB-1 message will include a list of vertical coordinate parameters ("PV") values corresponding to "A" and "B" values for either "rho" or "theta" levels. Level parameters 1..50 are the A(k) values and level parameters 51..100 are the B(k) values listed in Appendix C.

NetCDF

NetCDF data is of the "classic format" variant (compatible with the unidata NetCDF library version 3.6.x+)

Please ignore the accum_value metadata value. This parameter is not always set correctly for non-instantaneous fields.

Longitudes in the NetCDF files lie in the range [0.0 degrees, 360.0 degrees]. The NetCDF files somewhat misleadingly describe the valid range attribute for this dimension as -180 to +360.

Appendix B - Extended Field Descriptions

Extended description of multi-level fields

Par

NetCDF var

Detailed description

132.128

merid_wnd

Meridional (V) component of the wind velocity. [m s-1]

131.128

zonal_wnd

Zonal (U) component of the wind velocity. [m s-1]

Please note:

Fields in the above tables are listed in the same order as shown in the Parameters section of this document.

The nature of fields can not always be deduced easily from their NetCDF variable names. It is always safest to refer to the detailed description of a parameter.

Appendix C - Hybrid Height Levels

Level index

A [theta]

B [theta]

A [rho]

B [rho]

50

62918.64700000

0.00000000

59968.64960862

0.00000000

49

57018.64592538

0.00000000

54506.41131237

0.00000000

48

51994.17669936

0.00000000

49855.77951937

0.00000000

47

47717.38863124

0.00000000

45895.45337006

0.00000000

46

44073.51810888

0.00000000

42516.86044718

0.00000000

45

40960.20278548

0.00000000

39623.48354624

0.00000000

44

38286.76430699

0.00000000

37130.13081919

0.00000000

43

35973.49733139

0.00000000

34962.21850189

0.00000000

42

33950.94596425

0.00000000

33055.09139267

0.00000000

41

32159.23052923

0.00000000

31353.28041235

0.00000000

40

30547.32400360

0.00000000

29809.82308279

0.00000000

39

29072.32216198

0.00000000

28385.54625438

0.00000000

38

27698.77663865

0.00000000

27048.35512528

0.00000000

37

26397.93990378

0.00000000

25772.52855260

0.00000000

36

25147.11090956

0.00000000

24537.99548795

0.00000000

35

23928.88006635

0.00000000

23329.66174791

0.00000000

34

22730.44342948

0.00000000

22136.66757401

0.00000000

33

21542.89171854

0.00000000

20951.69552760

0.00000000

32

20360.49933666

0.00000000

19770.26580102

0.00000000

31

19180.03226537

0.00000000

18590.01265313

0.00000000

30

17999.99933275

0.00000000

17409.99859611

0.00000000

29

16819.99785946

0.00114844

16249.99896069

0.00443934

28

15680.00006192

0.00987402

15130.00300104

0.01715025

27

14579.99964829

0.02642257

14049.99813342

0.03724613

26

13520.00291041

0.04992301

13009.99694156

0.06387166

25

12499.99726457

0.07953637

12009.99942547

0.09620329

24

11520.00158636

0.11445445

11049.99929327

0.13344941

23

10579.99700018

0.15390196

10130.00283684

0.17484966

22

9680.00238163

0.19713380

9249.99747243

0.21967618

21

8819.99885511

0.24343826

8410.00207566

0.26723123

20

7999.99900435

0.29213375

7609.99777092

0.31685076

19

7220.00282936

0.34257095

6849.99714194

0.36790059

18

6479.99774639

0.39413311

6130.00018873

0.41977890

17

5780.00263106

0.44623297

5450.00061941

0.47191604

16

5119.99860776

0.49831768

4809.99843399

0.52377363

15

4499.99826022

0.54986368

4209.99992433

0.57484440

14

3920.00158845

0.60038002

3649.99879858

0.62465380

13

3380.00230057

0.64940801

3130.00134858

0.67275785

12

2880.00039659

0.69652009

2650.00128248

0.71874541

11

2420.00216837

0.74131978

2209.99860028

0.76223643

10

2000.00132405

0.78344345

1809.99959384

0.80288190

9

1619.99786363

0.82255856

1449.99797130

0.84036573

8

1279.99807897

0.85836360

1130.00002453

0.87440222

7

980.00197008

0.89058931

849.99946165

0.90473866

6

719.99695322

0.91899952

610.00257453

0.93115265

5

500.00190398

0.94338632

410.00307131

0.95345487

4

319.99794678

0.96357758

250.00095199

0.97148696

3

179.99766534

0.97942938

130.00250843

0.98512152

2

80.00105966

0.99083087

50.00144877

0.99426426

1

20.00183788

0.99770358

9.99777301

0.99885182

Notes:

[theta] and [rho] denote the level types on which the parameters A and B are valid.

A values represent the height of the hybrid levels above the geoid assuming zero topography.

B values represent the fraction of the topographic height included in the hybrid levels.

The height of a hybrid-height level can be calculated via:

z(k, i, j) = A(k) + B(k)*topo(i, j)
where k denotes the level index,
z(k, i, j) is the height above the geoid at grid-point (i, j, k),
topo(i, j) is the height of the surface above the geoid at (i, j) and
A(k) & B(k) are the values of A and B at level k.